The speed range of an electrical machine is determined by many factors, including but not limited to, pole number, turns per phase and supply voltage. Brushless electronically commutated synchronous motors include permanent magnet brushless dc and brushless ac, synchronous reluctance, flux switching and switched reluctance motors.
An electronically commutated motor relies on knowledge of the position of the rotor to correctly excite particular phase windings in the motor to deliver torque of the required magnitude and direction. This rotor position information can be obtained by using methods to detect the rotational EMF in the motor windings. Such methods are known as sensorless commutation methods because they do not rely on sensors to detect the angular position of the shaft of the motor. These methods are common and work effectively at high speeds but deteriorate at low speeds. This is due to the decrease in the value of the rotational EMF, reducing the signal to noise ratio to a level which makes it difficult to use the data for closed loop position control or electronic commutation. In such motors it is common to drive the motor in the lower part of the speed range by exciting the stator phase windings with maximum current at a driven frequency. This method is known in the prior art as driving the motor in an open loop manner. Open loop methods are commonly used to start a motor, increasing the frequency until the EMF can be detected and a reliable transition to sensorless control can be made. Such systems are however “unintelligent” in that the transition to the sensorless method will typically be made at a fixed speed. This can lead to destabilising effects since the decision does not take account of the effect of the load. A machine driven in open loop will typically have an excess of current relative to the current needed to run with closed loop electronically commutated control. Once the electronic commutation becomes dependent on knowledge of the machine position, the torque can increase rapidly as the higher open loop current levels are brought into correct alignment with the motor EMF. The rapid increase in torque can cause a speed oscillation.